Pressurized-water nuclear reactors incorporate a primary circuit, in which the pressurized water cooling the fuel assemblies of the reactor core circulates. The primary circuit communicates with the inner volume of the vessel containing the core and has primary-fluid circulation pumps, steam generators and a pressurizer, which are connected by means of large-diameter and pressure-resistant pipelines. The primary fluid of the reactor can also be made to circulate in certain auxiliary circuits which allow it to be treated and to have its physical or chemical characteristics modified.
During its circulation in the primary circuit or in the auxiliary circuits, the cooling fluid comes in contact with many components, most of which are made of or covered with a nickel alloy making it possible to limit the extent to which they are attacked by the primary fluid. However, some components, such as the seats of valves or of cocks, or even certain portions of piping, experience a certain wear, with the result that the primary fluid becomes laden with particles of very small dimensions which become detached from these components. These particles tend to circulate together with the primary fluid and therefore pass through the reactor core, where they are subjected to intense neutron bombardment, the effect of which is to activate them. In particular, wear-resistant alloys containing a certain proportion of cobalt cause highly activated particles to occur. These particles accumulate in certain parts of the reactor components, and this presents problems which are very difficult to solve during the maintenance operations on the reactor, because these operations require preliminary decontamination phases which are very difficult to carry out.
On the other hand, the make-up water and additives introduced into the primary fluid by means of an auxiliary circuit, such as the volumetric and chemical monitoring circuit, likewise contains solid particles of various origins which are activated when the primary fluid passes through the reactor core.
It is therefore necessary to treat the primary fluid periodically or continuously to reduce the content of activated or activatable particles therein. These particles have a mean diameter of 0.5 microns, a considerable proportion of particles having a diameter of the order of 0.1 microns. These particles can also occur in colloidal form, i.e., in the form of a non-crystallized gel.
The fluid therefore has to be purified by means of a process, such as ultrafiltration, more particularly by means of hot ultrafiltration, since the solubility of the pollutant products is higher when cold than when hot.
It has therefore been proposed to use hot-ultrafiltration processes on the primary fluid at its operating temperature and pressure in the reactor, so that it could be purified during the operation of the reactor. Such a process was described in French Pat. No. 83-15130 in the name of the present assignee. This patent also describes an ultrafiltration device which can be inserted in the circulation of the primary fluid, within the containment shell of the reactor. Such an ultrafiltration device, through which a fluid passes at a temperature in the neighborhood of 320.degree. and at a pressure of the order of 155.times.105 Pa, has a pressure-resistant casing of very great thickness, which is made of a material resistant to the corrosive action of the primary fluid and its additives. However, such an ultrafilter is designed in one piece, the ultrafiltration wall consisting of tubes fastened at their ends to tube plates, themselves welded to the pressure-resistant casing of the ultrafilter. This gives rise to difficulties in maintaining and repairing the filter, for example when the tubes forming the ultrafiltration wall are clogged or damaged.
On the other hand, installing the ultrafilter in the containment shell of the reactor containing the primary circuit and taking it off from the primary circuit or from an auxiliary circuit of the reactor makes it necessary to carry out complex operations and alters the construction program for the nuclear reactor. Likewise, replacing the ultrafilter of a reactor which has been in operation requires lengthy and difficult operations both during the removal of the filtr to be replaced, the maerials of which are activated, and during the fitting of the new filter.
These operations are all the more complex because the ultrafilter is inserted in an ultrafiltration circuit incorporating components, such as pumps, heat exchangers, depressurizers and valves. Both these components and the connecting pipework have to be carefully heat-insulated and placed in protected spaces within the containment shell, since they are intended for receiving a liquid which is at a very high temperature and a very high pressure and which transports activated particles. In particular, great precautions must be taken in the design and assembly of the part of the ultrafiltration circuit receiving the concentrate which usually incorporates a pump and a heat exchanger for cooling the concentrate.